It is an important constituent of air because it has the ability to absorb heat and thus keep the atmosphere warm, thereby, balancing the heat of the earth.

Water Vapour is the most significant component of the atmosphere as far as its effect on weather is concerned although its quantity varies considerably from practically none (0) to up to about 4% by volume.

Water Vapour is the source of all clouds and precipitation(rain, hail storm etc.). Water vapour, like carbon dioxide, has the ability to absorb heat energy. It also reflect incoming insolation.

Dust intercepts and reflect incoming insolation.

The polluted particles present in the air not only absorb larger amount of insolation but also greatly absorb the terrestrial radiation.

Dust in the atmosphere contributes to the red and orange colour of sunrise and sunset.

Layers of the Atmosphere

There are five distinct layers of the atmosphere - (a) Troposphere (b) Stratosphere (c) Mesosphere (d) Thermosphere and (e) Exosphere

Troposhere

This is the first layer of the atmosphere. It extends to a height of 18 km at the equator and 8 km at the poles.

In this layer temperature decreases with height. This is due to the fact that the density of air decreases with height and so the heat absorbed is less. It contains more than 90% of gases in the atmosphere.

Since most of the water vapour form clouds in this layer, all weather changes occur in the troposphere("tropo" means "change").

The height at which the temperature stops decreasing is called tropopause. Here the temperature may be as low as -58 degree Celsius.

Stratosphere

This is the second layer of the atmosphere. It extends from the tropopause to about 50 km.

Temperature increases due to the absorption of the ultraviolet radiations of the Sun by Ozone present in this layer. The temperature slowly increases to 4 degree celsius.

This layer is free from clouds and associated weather phenomena. Hence, it provides ideal flying conditions for large jet planes.

At about 50 km the temperature begins to fall again. This marks the end of the stratosphere. The end of the stratosphere is called the Stratopause.